New amiloride analogue as hapten to raise anti-amiloride antibodies

A new amiloride analogue, "amiloride-caproic acid," was synthesized, coupled to albumin, and used as a hapten to raise anti-amiloride antibodies in rabbits. The antibodies were affinity purified with an amiloride affinity column and characterized. Binding studies using [3H]benzamil showed a dissociation constant of 0.8 nM. Amiloride and amiloride-caproate inhibited [3H]benzamil binding; epsilon-guanidinocaproic acid showed no inhibition. Anti-amiloride antibodies reversed the inhibition by amiloride of sodium transport across toad urinary bladder. Anti-amiloride antibodies and an amiloride affinity column should provide useful tools for the characterization of the epithelial sodium channel.

Plasticity of functional epithelial polarity

The fundamental characteristics that allow vectorial transport across an epithelial cell are the differential sorting and insertion of transport proteins either in the apical or the basolateral plasma membrane, and the preferential association of endocytosis and exocytosis with one or the other pole of the cell. Asymmetrical cellular structure and function, being manifestations of terminal differentiation, might be expected to be predetermined and invariant. Here we show that the polarity of transepithelial H+ transport, endocytosis and exocytosis in kidney can be reversed by environmental stimuli. The HCO3- secreting cell in the cortical collecting tubule is found to be an intercalated cell possessing a Cl-/HCO3- exchanger in the apical membrane and proton pumps in endocytic vesicles that fuse with the basolateral membrane; the H+-secreting cell in the medullary collecting tubule has these transport functions on the opposite membranes. Further, the HCO3- -secreting cell can be induced to change its functional polarity to that of the H+-secreting cell by acid-loading the animal.

Carbon dioxide causes exocytosis of vesicles containing H+ pumps in isolated perfused proximal and collecting tubules

In the turtle bladder it has recently been shown that CO2 stimulates H+ secretion, at least in part, by causing fusion of vesicles enriched in H+ pumps with the luminal plasma membrane. To test for the presence of this mechanism in the kidney we perfused collecting ducts and proximal straight tubules on the stage of an inverted epifluorescence microscope with fluorescein isothiocyanate dextran (70,000 mol wt) in CO2-free medium. After washout we noted punctate fluorescence in endocytic vesicles in some collecting ducts and in all proximal straight tubule cells. More cells took up fluorescent dextran in outer medullary than in cortical collecting ducts. Using the pH dependence of the excitation spectrum of fluorescein we found the pH of the vesicles to be acid (approximately pH 6). Addition of proton ionophores increased vesicular pH by 0.6 +/- 0.1 U, suggesting that the acidity of the vesicles was caused by H+ pumps. CO2 added to the medium (25 mmHg, pH 7.6 at 37 degrees C) reduced fluorescence intensity by 24 +/- 5% in cortical collecting ducts, 27 +/- 5% in medullary collecting ducts, and 25 +/- 5% in proximal straight tubules. Since this effect was prevented by the prior addition of colchicine to the bath, we believe that CO2 caused a decrease in cytoplasmic fluorescence by stimulating exocytotic fusion of the vesicles and thereby secretion of fluorescent dextran. This exocytotic fusion also occurred when tubules that were loaded with fluorescent dextran at a pCO2 of 37 mmHg were exposed isohydrically to a pCO2 of 114 mmHg; the mean decrease was 53 +/- 4%. We conclude that some cells in the collecting ducts and all cells in the proximal straight tubule incorporate fluorescent dextran into the apical cytoplasmic vesicles and acidify them with H+ pumps. CO2 causes fusion of these vesicles with the luminal membrane, but whether CO2 stimulates H+ secretion by increasing the number of functioning H+ pumps remains to be determined.

Carbon-dioxide-induced exocytotic insertion of H+ pumps in turtle-bladder luminal membrane: role of cell pH and calcium

The contents of endocytic vesicles and other intracellular organelles (such as Golgi and microsomes) are acidified by an electrogenic proton-translocating ATPase that is remarkably similar to that found in urinary epithelia. We recently found that the number of H+ ATPases in the apical plasma membrane of these epithelia is regulated by exocytotic insertion of endocytic vesicles whose membranes contain this H+ pump. Carbon dioxide, a major stimulus for urinary acidification, causes rapid fusion of these vesicles with the luminal membrane, thereby inserting these pumps there and increasing the rate of net transepithelial H+ secretion; CO2 also inhibits endocytic retrieval of the pumps from the luminal membrane. Such reciprocal regulation of endocytosis and exocytosis by a physiological modulator makes this system particularly attractive for studying the cellular events regulating membrane fusion. Here we present evidence that CO2 induces exocytosis by a cascade of events, the first step of which is cytoplasmic acidification. Cell acidification then increases calcium activity, which causes the fusion event.

An electrogenic proton-translocating adenosine triphosphatase from bovine kidney medulla

Urinary acidification in the mammalian collecting tubule is similar to that in the turtle bladder, an epithelium whose H+ secretion is due to a luminal proton-translocating ATPase. We isolated a fraction from bovine renal medulla, which contains ATP-dependent proton transport. H+ transport was found to be electrogenic in that its rate was reduced by a membrane potential. H+ transport activity was inhibited by N-ethyl maleimide and dicyclohexyl carbodiimide, but not by oligomycin or vanadate; its activity did not depend on the presence of potassium, differentiating this ATPase from the mitochondrial F0-F1 ATPase and the gastric H+-K+ ATPase. H+ transport activity had a specific substrate requirement for ATP, distinguishing this pump from the lysosomal H+ ATPase, which uses guanosine or inosine triphosphate as well. The distribution of this H+ pump on linear sucrose density gradient was different from that of markers of lysosomes and basolateral membranes. These results show that the kidney medulla contains an H+ -translocating ATPase different from mitochondrial, gastric, and lysosomal proton pumps, but similar to the turtle bladder ATPase.

Proton-translocating adenosinetriphosphatase in rough and smooth microsomes from rat liver

Rat liver smooth and rough microsomal membranes exhibit an ATP-dependent H+ transport which can be inhibited by sulfhydryl reagents and dicyclohexylcarbodiimide but is resistant to oligomycin. On the basis of inhibitor sensitivities and substrate specificities, this H+ pump was found to be different from that of mitochondria, lysosomes, gastric H+-K+-ATPase, and yeast plasma membrane H+-ATPase but to resemble that of endocytic vesicles and the H+ pump responsible for urinary acidification. The transport process is accelerated by valinomycin in the presence of potassium, suggesting that it is an electrogenic pump. The same fractions were enriched in an ATPase with inhibitor sensitivities similar to those of the transport activity. It is possible that the proton electrochemical gradients generated by this pump may play a role in the translocation of proteins and sugars, two of the major functions of these structures.

Golgi membranes contain an electrogenic H+ pump in parallel to a chloride conductance

Rat liver Golgi vesicles were isolated by differential and density gradient centrifugation. A fraction enriched in galactosyl transferase and depleted in plasma membrane, mitochondrial, endoplasmic reticulum, and lysosomal markers was found to contain an ATP-dependent H+ pump. This proton pump was not inhibited by oligomycin but was sensitive to N-ethyl maleimide, which distinguishes it from the F0-F1 ATPase of mitochondria. GTP did not induce transport, unlike the lysosomal H+ pump. The pump was not dependent on the presence of potassium nor was it inhibited by vanadate, two of the characteristics of the gastric H+ ATPase. Addition of ATP generated a membrane potential that drove chloride uptake into the vesicles, suggesting that Golgi membranes contain a chloride conductance in parallel to an electrogenic proton pump. These results demonstrate that Golgi vesicles can form a pH difference and a membrane potential through the action of an electrogenic proton translocating ATPase.

Regulation of proton transport in urinary epithelia

In urinary epithelia, like the turtle bladder, protons are transported by a H+ translocating ATPase located in the luminal membrane. We have recently discovered that the H+ pump is stored in small vesicles that lie underneath the luminal membrane. CO2, a major regulator of H+ transport causes these vesicles to fuse with the membrane thereby inserting more H+ pumps. We have now isolated these vesicles from the turtle bladder and from beef kidney medulla. Based on inhibitor sensitivity and substrate specificity this proton translocating ATPase is different from the mitochondrial F0-F1 ATPase, yeast plasma membrane and the gastric H+,K+-ATPase. Solubilization and reconstitution of the enzyme into liposomes shows retention of transport activity and inhibitor sensitivity.

Calcium reduces the sodium permeability of luminal membrane vesicles from toad bladder. Studies using a fast-reaction apparatus

Regulation of the sodium permeability of the luminal membrane is the major mechanism by which the net rate of sodium transport across tight epithelia is varied. Previous evidence has suggested that the permeability of the luminal membrane might be regulated by changes in intracellular sodium or calcium activities. To test this directly, we isolated a fraction of the plasma membrane from the toad urinary bladder, which contains a fast, amiloride-sensitive sodium flux with characteristics similar to those of the native luminal membrane. Using a flow-quench apparatus to measure the initial rate of sodium efflux from these vesicles in the millisecond time range, we have demonstrated that the isotope exchange permeability of these vesicles is very sensitive to calcium. Calcium reduces the sodium permeability, and the half-maximal inhibitory concentration is 0.5 microM, well within the range of calcium activity found in cells. Also, the permeability of the luminal membrane vesicles is little affected by the ambient sodium concentration. These results, when taken together with studies on whole tissue, suggest that cell calcium may be an important regulator of transepithelial sodium transport by its effect on luminal sodium permeability. The effect of cell sodium on permeability may be mediated by calcium rather than by sodium itself.

The proton translocating ATPase responsible for urinary acidification

H+ secretion by the turtle urinary bladder is produced by a proton pump located in the luminal membrane. We show that a microsomal fraction of these cells contains an electrogenic proton translocating ATPase that is inhibited by dicyclohexylcarbodiimide but not oligomycin or vanadate. On a sucrose density gradient, this ATPase co-migrated with luminal membranes labeled with concanavalin A, but was separate from a lysosomal marker. This enzyme is therefore the H+ ATPase that causes urinary acidification.

Exocytosis regulates urinary acidification in turtle bladder by rapid insertion of H+ pumps into the luminal membrane

Urinary acidification by the turtle bladder is due to a H+-ATPase that is located in the luminal membrane. The rate of H+ transport is stimulated by an increase in the ambient CO2. Using the fluorescent dye acridine orange, we showed that the mitochondria-rich cell of this equilibrium contains vesicles whose internal pH is acidic. We measured the pH of these vesicles by using endocytosed fluorescein isothiocyanate-labeled dextran and found it to be near 5.0. The pH increased after treatment with protonophores or metabolic inhibitors, suggesting that it was due to a H+ pump rather than to a Donnan effect. In bladders preloaded with fluorescent dextran, CO2 stimulated exocytosis and H+ transport measured simultaneously in the same bladder. The increase in the H+ current correlated well with the extent of exocytosis, and both were inhibited by pretreatment with colchicine. We conclude that the turtle bladder contains an intracellular reserve of vesicles containing H+ pumps and CO2 stimulates rapid fusion of these vesicles with the luminal membrane with consequent insertion of H+ pumps, thereby stimulating H+ secretion across the whole epithelium.

Regulation of the sodium permeability of the luminal border of toad bladder by intracellular sodium and calcium: role of sodium-calcium exchange in the basolateral membrane

Sodium movement across the luminal membrane of the toad bladder is the rate-limiting step for active transepithelial transport. Recent studies suggest that changes in intracellular sodium regulate the Na permeability of the luminal border, either directly or indirectly via increases in cell calcium induced by the high intracellular sodium. To test these proposals, we measured Na movement across the luminal membrane (th Na influx) and found that it is reduced when intracellular Na is increased by ouabain or by removal of external potassium. Removal of serosal sodium also reduced the influx, suggesting that the Na gradient across the serosal border rather than the cell Na concentration is the critical factor. Because in tissues such as muscle and nerve a steep transmembrane sodium gradient is necessary to maintain low cytosolic calcium, it is possible that a reduction in the sodium gradient in the toad bladder reduces luminal permeability by increasing the cell calcium activity. We found that the inhibition of the influx by ouabain or low serosal Na was prevented, in part, by removal of serosal calcium. To test for the existence of a sodium-calcium exchanger, we studied calcium transport in isolated basolateral membrane vesicles and found that calcium uptake was proportional to the outward directed sodium gradient. Uptake was not the result of a sodium diffusion potential. Calcium efflux from preloaded vesicles was accelerated by an inward directed sodium gradient. Preliminary kinetic analysis showed that the sodium gradient changes the Vmax but not the Km of calcium transport. These results suggest that the effect of intracellular sodium on the luminal sodium permeability is due to changes in intracellular calcium.

Metabolic pathways coupled to H+ transport in turtle urinary bladder

Active H+ transport in the turtle urinary bladder is mediated by an ATPase. Although the source of ATP is usually mitochondrial oxidative phosphorylation, it is possible because of intracellular compartmentalization or cellular heterogeneity that one metabolic pathway exclusively provides ATP to the pump. To examine this we performed several types of experiments. In one, the coupling between the rate of transport and the rate of oxidation of 14C-labeled substrates was studied. We found that there was coupling between H+ transport and glucose, butyrate oleate, and beta-OH-butyrate oxidation. In another set of experiments we depleted turtle bladders of their endogenous substrates and tested the effect of a number of substrates on the rate of transport. We found that glucose, pyruvate, lactate, actetate, butyrate and beta-OH butyrate all stimulated H+ transport. In a third set of experiments we found no coupling between H+ transport and lactate production. Finally, we found that reduction of H+ transport by mucosal acidification resulted in an increase in epithelial cell ATP concentrations and a decrease in ADP levels. These results suggest that the H+ pump receives its ATP from carbohydrate and fatty acid oxidation. The changes in ATP and ADP levels provide an initial explanation for the coupling of H+ transport to the rate of cellular oxidative metabolism.

H+/ATP stoichiometry of proton pump of turtle urinary bladder

Urinary acidification in the turtle urinary bladder is due to a reversible proton-translocating ATPase. To estimate the H+/ATP stoichiometry of this pump, we measured the delta G'ATP in the epithelial cells and the maximum e.m.f. generated by the pump. The latter is the maximal transepithelial electrochemical gradient for protons placed across the epithelium that is needed to nullify the rate of transport and averaged 179 +/- 7 mV. The delta G'ATP averaged 50.1 kJ/mol. The H+/ATP stoichiometry of these bladders was 2.92 +/- 0.1. In other experiments, the bladders were poisoned by iodoacetate and cyanide and a variable transepithelial electrochemical gradient for protons was placed across them. It was noted that ATP synthesis occurred at a transepithelial electrochemical gradient for protons greater than 120 mV. The delta G'ATP in other bladders treated identically averaged 40.0 kJ/mol, giving a H+/ATP stoichiometry of 3.4 +/- 0.1. We conclude that the H+/ATP stoichiometry of the proton pump of turtle urinary bladder is approximately 3.

Urinary acidification in turtle bladder is due to a reversible proton-translocating ATPase

Adverse proton electrochemical gradients (delta muH) applied across the turtle urinary bladder decrease active H+ transport in this epithelium. A delta muH of 180 mV abolishes both transport and its tightly coupled metabolic reaction. Larger gradients should, in theory, reverse the direction of H+ transport and the metabolic reaction leading to synthesis of ATP if the pump is an ATPase, or cause an increase in the oxidized state of a redox pair if it is a redox pump. To distinguish between these two possibilities, we measured ATP levels in epithelial cells that were poisoned to inhibit cellular mechanisms of ATP synthesis. At delta muH of 120 mV or less no ATP synthesis was found. At delta muH of greater than 120 mV there was a linear increase in ATP synthesis. Dinitrophenol, a H+ carrier, prevented synthesis at delta muH of 310 mV. Dicyclohexylcarbodiimide, an inhibitor of H+ transport that works at the cell surface, prevented ATP synthesis at delta muH of 310 mV. These results demonstrate that a reversible proton-translocating ATPase in the mucosal border of the bladder is the H+ pump responsible for urinary acidification.

Interaction between sodium and chloride transport in canine tracheal mucosa

Canine tracheal mucosae were dissected and mounted as flat sheets in Ussing chambers. Unidirectional isotope fluxes of 22Na and 36Cl were performed across paired mucosae from the same animal. The average spontaneous potential difference was 42 + 1.2 mV (mean +/- SE) lumen negative. The short-circuit current (SCC) 3.09 +/- 0.36 mueq/cm2.h was accounted for by a net Cl secretion of 2.46 +/- 0.26 mueq/cm2.h toward the mucosa and net Na absorption of 0.46 +/- 0.13 mueq/cm2.h toward submucosa. Removal of Cl depressed SCC but had no effect on unidirectional or net Na transport (n = 7). By contrast, removal of Na (n = 6) or the addition of ouabain (n = 7) abolished net Cl secretion and greatly reduced SCC. Theophylline (n = 6) added to the submucosal bath no significant effect on Na transport but stimulated SCC and Cl secretion, suggesting hormonal regulation of Cl transport. The results suggest that the active transport of Na and Cl in this epithelium occur by electrically conductive pathways, i.e., the transport is "electrogenic." Further it appears that Na transport is independent of the presence of Cl but that Cl transport depends on some parameter of active Na transport.

H + transport in urinary epithelia

This review of urinary acidification is primarily based on studies in isolated epithelia such as the turtle bladder. Despite the lack of unambiguous proof, the wealth of indirect evidence suggests that the cause of bicarbonate absorption is H+ secretion into the lumen. The mechanisms that regulate H+ transport are discussed. The electrochemical gradient for protons across the membrane is found to be the most fundamental regulator not only of passive movement but also of active transport. CO2 and aldosterone stimulate H+ transport, the latter by a mechanism apparently separate from the effect of this hormone on sodium transport. Although carbonic anhydrase activity is important for optimal function of the H+ pump, the results with carbonic anhydrase inhibitors need to be interpreted with caution. The evidence for Na:H exchange is reviewed and found to be not very persuasive, The metabolic pathways that fuel H+ transport are found to be all the major energy-yielding reactions in the cell, but particular prominence is given to the new discovery of the role of the pentose shunt in energizing transport. Finally, I discuss the important role H+ transport in energy transduction in subcellular organelles.

Effect of aldosterone on the coupling between H+ transport and glucose oxidation

The mode of action of aldosterone on the energetics of H+ transport in the turtle bladder was examined with the rate of glucose oxidation as an index of the metabolic activity of the epithelium (we show that H+ transport is not coupled to fatty acid oxidation). Within 6 h of addition of aldosterone H+, transport increased; so did glucose oxidation. The amount of H+ transport per mole of 14CO2 produced from glucose oxidation was 15.6 eq-mol-1 in the control hemi-bladder, while in the aldosterone-treated bladder it was 13.6, delta = 2.0+/-4.0 (n = 6). However, in bladders exposed to aldosterone for 20 h, the relation of transport to glucose oxidation was significantly altered: control 10.8, aldosterone 16.4, delta = 4.5+/-2.5, P less than 0.02, n = 7. The slope of H+ transport on the applied electrochemical gradient was steeper during both short- and long-term incubations. However, the maximum gradient necessary to nullify the net rate of secretion was unaltered in both experiments. Evidence is presented that aldosterone does not alter the passive backflux into the cell. In five additional experiments where aldosterone produced no significant stimulation of H+ transport, no change was noted in any of the metabolic or transport characteristics measured, suggesting that the alterations discussed above are dependent on the stimulation of H+ transport by the hormone. These results, along with some thermodynamic considerations, suggest that the effect of aldosterone is primarily exerted on the transport process rather than on metabolism. Further, it appears that prolonged stimulation of transport work leads to secondary alterations in the metabolic pathways reminiscent of the changes that occur in skeletal muscles of athletes undergoing physical conditioning.

Active H+ transport in the turtle urinary bladder. Coupling of transport to glucose oxidation

The turtle urinary bladder acidifies the contents of its lumen by actively transporting protons. H+ secretion by the isolated bladder was measured simultaneously with the rate of 14CO2 evolution from [14C]glucose. The application of an adverse pH gradient resulted in a decline in the rate of H+ secretion (JH) and in the rate of glucose oxidation (JCO2). The changes in JH and JCO2 were linear functions of the pH difference across the membrane. Hence, JH and JCO2 were linearly related to each other. The slope, deltaJH/deltaJCO2 was found to be similar in half-bladders from the same animal but was seen to vary widely in a population of turtles. To investigate the effect of pH gradients on deltaJH/deltaJCO2, two experiments were performed in each of 14 hemibladders. In one, JH and JCO2 were altered by changing the luminal pH. In the other, they were altered by changing the ambient pCO2 while the luminal pH was kept constant. The average slope, deltaJH/deltaJCO2, in the presence of pH gradients was 14.45 eq-mol-1. In the absence of gradients in the same hemibladders it was 14.72, delta = 0.27 +/- 1.46. The results show that H+ transport is organized in such a way that leaks to protons in parallel to the pump are negligible. Analysis of the transport system by use of the Essig-Caplan linear irreversible thermodynamic formalism shows that the system is tightly coupled. The degree of coupling, q, given by that analysis was measured and found to be at or very near the maximum theoretical value.